Signal processing apparatus and method

ABSTRACT

Signal processing, particularly correlation and auto-correlation, is effected using a &#34;light panel&#34;. The light panel consists of (a) an array of two-terminal light-emitting or light-modulating elements (10), the optical state of each element being changed from a first state to a second state when a voltage difference is applied across its terminals, and (b) a complementary array of photo-diodes (12), each arranged to receive light from a respective element of the array of light-emitting or light-modulation elements (10). The interconnection between the two arrays may be simply an optical lens (11). For auto-correlation, each array may be a linear array, with the signal from a single signal source applied to the terminals of each element (10) both instantaneously and after a predetermined time delay. For cross-correlation, each element (10) will be located at a cross-over point of an m x n matrix of conductors (insulated from each other) with its terminals connected to respective conductors at the cross-over point. When the light panel is used for correlation, the photodiodes act as charge integrators. Uses of the apparatus and method include image processing, image analysis and speckle interferometry.

TECHNICAL FIELD

This invention concerns signal processors. In a particular application,it concerns correlators and auto-correlators which use an array of lightemitting or light modulating elements operating as "EXCLUSIVE OR" or"NEGATED-EXCLUSIVE-OR" logic elements, the optical output from such anarray being processed to provide correlation information.

BACKGROUND ART

The technique of one-bit digital processing is a well-accepted tool ofscience. For example, its implementation in radio astronomy spectroscopywas described by B. F. C. Cooper in his paper entitled "AutocorrelationSpectrometers", which was published in "Methods of ExperimentalPhysics", Volume 12, Part B, Edited by M. L. Meeks (Academic Press,1976), pages 289-298. Various forms of correlator have been produced forsuch correlation. These have up to several thousand channels and clockrates up to several tens of megahertz. Larger and faster correlators arenot produced, however, for with prior art components and arrangements,the larger correlators are found to possess inherent disadvantages intiming accuracy, bulk size and power dissipation.

SUMMARY OF THE INVENTION

One object of the present invention is the provision of means wherebyone-bit correlators which may be constructed so that they are large,fast, and without severe heat dissipation problems. Another objective isthe provision of one-bit correlators which are simple to construct, andwhich can be made large in terms of number of channels, yet compact andwith low power dissipation.

These objectives are achieved by the construction of correlators usingwhat has been termed by the present inventor a "light panel". The lightpanel is an array of light emitting or light modulating elementsarranged in matrix form, operating as "EXCLUSIVE OR" ("XOR") or"NEGATED-EXCLUSIVE-OR" ("NEXOR") logic elements. The optical output fromthe elements of the light panel are processed by optical means, such asa complementary array of photodiodes.

According to the present invention, there is provided a correlator whichcomprises, in combination:

(a) an array of light emitting or light modulating elements, eachelement being a device which changes from a first optical state to asecond optical state when a voltage is applied across it, each elementbeing adapted to receive two voltage signals, the optical state of eachelement at any instant being dependent on the relative values of thevoltage signals; and

(b) a complementary array of photodiodes, each photodiode being adaptedto receive light from a respective one of said light emitting or lightmodulating elements.

If the array is a linear array, the correlator may be used as anautocorrelator, as will be shown below. For purposes other thanautocorrelation of a signal, the array will usually be in the form of atwo-dimensional array. Each light emitting or light modulating elementmay conveniently be a light emitting diode device and series resistorcombination, a neon-discharge or other plasma-discharge lamp (preferablywith a series current-limiting capacitor or resistor), a liquid crystal,a deuterated potassium di-hydrogen phosphate (DKDP) or other Pockel'seffect device, a vacuum fluorescent device, an electroluminescentdevice, or any other suitable device having the appropriate properties.

If, as indicated above, the array of light emitting or light modulatingelements is a one-dimensional array (usually a linear array), and one ofthe signals applied to the elements is common and the other signals arethe outputs of shift registers or delay lines to which the common signalsource has also been connected, an autocorrelator is formed.

As also noted above, the present invention will provide larger one-bitcorrelators. In this case, the light emitting or light modulatingelements of the array will be located at the cross-over points of amatrix formed by a plurality of linear electrical conductors, eachconductor being insulated from each other conductor, the elements beingconnected across the conductors at the respective cross-over points.

The electrical conductors of such a light panel need not be formed as arectangular matrix (though in practice this will often be the case), butcan be formed in any required configuration. For example, for aparticular correlation, it may be convenient to form the matrix in aconfiguration which is representative of a polar diagram, or the spatialfrequency plane of a circular array of receiving aerials (such as thesolar radioheliograph described by N. R. Labrum, D. J. McLean and J. P.Wild in "Methods of Computational Physics" Volume 14, page 1, 1975).

A feature which will commonly be present in correlators constructed inaccordance with the present invention, is means to focus the emitted ormodulated light signals from the elements of the array on to respectivephotodiodes in the complementary array of photodiodes.

The correlation method of the present invention may also be extended toobserving the optical state of each element of the array oflight-emitting or light-modulating elements by focusing the lightemitted or modulated by the elements of the array on to a complementaryarray of photodiodes.

The outputs from the photodiodes may be displayed or processed in anyconvenient manner.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a linear array of light-emitting diodes (a "lightpanel") imaged on to a linear array of photodiodes, and connected forautocorrelation of a one-bit quantized signal;

FIG. 2 illustrates the steps usually adopted in one-bit quantization ofan analogue signal;

FIG. 3 is a diagram of a light panel construction for a complete one-bitautocorrelator with facilities for indication of true "XOR" logicsignals and the correlation or the anti-correlation function;

FIG. 4 illustrates the construction of a light panel for a one-bitcross-correlator in which four input signals are compared with each often other input signals; and

FIG. 5 is a diagram of a light panel for an m×n correlator, with choiceof correlation or anti-correlation.

MODES OF CARRYING OUT THE INVENTION

It is well-known that a light emitting diode device (LED) emits lightwhen a potential difference of the correct polarity is applied acrossit. Other devices exhibit a similar property. It has not previously beenappreciated that if binary digital signals of logic state 1 and 0 areapplied to the LED, the LED will flash whenever the logic states on thetwo sides are different and of the correct polarity. Thus the LEDperforms a logical EXCLUSIVE-OR (XOR) operation on the two signals. Dueto the unidirectional properties of a LED, the true XOR signal isobtained when each pair of signals is applied twice to the LED duringthe time for which the signals are sampled, the second time with thesignal polarity reversed. If (a) a series resistor is used to limit thecurrent through the LED and (b) the binary digital signals are arrangedto be either a constant voltage or zero (corresponding to logic states 1and 0) and are applied for a constant sample time, then the mean lightlevel from the LED is an accurate measure of the number of times thelogic states of the two signals are anti-coincident. The presentinvention utilises this property of LEDS (and other devices), as will beseen.

Referring now to FIG. 1, a linear array of light emitting diodes 10 islocated so that the light output of each diode is focused by lens 11 onto a corresponding photodiode in a complementary linear array ofphotodiodes 12. One terminal of each LED 10 is connected to an incomingquantized signal S. The other terminals of the LEDs 10 are connectedthrough respective limiting resistors R to the signal S after it hasbeen delayed by a time which is a multiple of "τ". If "τ" is the timeseparation of the successive samples of an electrical signal whichproduce the quantized signal S, each LED 10 compares the signal S withthe signal S that existed an integral number of sample periodspreviously. Each LED thus performs an autocorrelation function, andflashes if there is anti-correlation at the respective delay period (andif the polarity is correct to cause it to flash). To indicatecorrelation (that is, the logical NEXOR function), all that is necessaryis for the logical inverse of the signal S to be applied to the commonLED terminals.

As already noted, the light flashes from each LED 10 are focused by lens11 on to respective photodiodes 12. Typical commercially availablephotodiodes saturate at about 10⁷ photoelectrons per diode. Thephotodiode integrates received photoelectrons until saturation isreached. Choosing the operating conditions to be such that thephotodiodes 12 generate about 10 electrons each time the LED 10 flashesfor the duration of one sample interval, any photodiode 12 in the arraycan receive photoelectrons from about 10⁶ flashes before it saturates.Thus the photodiode array read-out is required at every 10⁶ sampleperiods or sooner.

Those skilled in this art will recognise that each LED and photodiodecombination is equivalent to one NEXOR circuit plus 20 stages of binarycounter (10⁶ is approximately 2²⁰). Such skilled persons will also beaware that read-out of photodiode arrays which have been constructed forsolid state TV displays can be effected at rates of less than 1microsecond per photodiode. Thus a 10,000 element photodiode array,requiring 10 milliseconds to read out, can be used with an equivalentarray of LEDs receiving digital one-bit binary signals, provided thatthe frequency of the binary digital signals does not exceed about 100MHz.

A typical derivation of the quantized signal S is illustrated in FIG. 2.In FIG. 2, an analogue electrical signal is shown at (a). It is sampledat the instants shown in (b), separated by a uniform clock period. Theoutput (quantized) signal shown at (c) is a positive value (typically +5volts for an LED array) when the analogue signal is positive, and iszero when the analogue signal is zero or negative at the moment ofsampling. Thus the signal S is a string of equal duration samples of 0or +5 volts (assuming the light panel comprises an LED array).

A light panel for a complete autocorrelation system is shown in FIG. 3.This arrangement overcomes the polarity limitation and enables bothcorrelation and anti-correlation to be measured. A logic inverting gate31 is connected into the common signal supply to the LEDs 10 to enablethe inverse of the binary signal S to be applied to the LEDs 10. Switch33 permits either correlation or anti-correlation to be displayed by theflash of an LED 10. Switch 33 also permits any inherent offset in thecorrelator, and any drift in the photodiode array, to be compensated, ifthe switch is operated so that correlation or anti-correlation isindicated for the other half of the period of measurement. Those skilledin this art will be well aware that for the normalised one-bitsituation, anti-correlation is simply 1-(correlation), so that equaltime spent measuring both allows their difference to be formed. It is,of course, assumed that the signal being analysed remains the samethroughout the period of measurement.

Polarity reversal of the signals applied to the LEDs 10 in thearrangement of FIG. 3 is achieved by the combination of logic invertinggates 32 and 35 with their respective associated switches 34 and 36.Switches 34 and 36 are synchronised, and each either (a) operate attwice the sample frequency to provide a polarity reversal after half thesample period, or (b) operate at the clock rate but with a quarter cyclephase adjustment.

It will be clear that with an associated photodiode array, linked asshown in FIG. 1, a light panel constructed as shown in FIG. 3 canproduce an autocorrelator in which

(a) true XOR functions for either correlation or anti-correlation can beobtained, by setting switch 33 to one position and operating switches 34and 36 to effect polarity reversals half-way through each sample period,and

(b) drift and offest effects in the photodiode array can be compensatedby choosing one setting for switch 33 and performing autocorrelation forhalf the total measurement time, then repeating the autocorrelation withthe switch 33 in its other setting, and differencing the twomeasurements of autocorrelation.

The light panel illustrated in FIG. 4 is designed for use in a one-bitcross-correlator in which the binary quantized signals from four sourcesare compared with the binary quantized signals from ten other sources.One example of the sort of situation in which this form ofcross-correlation would be used is in radio-astronomy, when the signalsreceived by the aerials on one arm of a T-shaped array are compared withthe signals received by the aerials in the other arm of the array.

The upper part of FIG. 4 illustrates the way in which signals fromsources A, B, C and D are applied to one terminal of a 4×10 array ofphotodiodes 40, each with its associated series resistor 41, and signalsfrom sources 0, 1, 2 . . . , 9 are applied to the other terminals of thearray. The lower part of FIG. 4 illustrates one physical form of thearray of photodiodes 40, and their connection to the respective signalsources through a matrix formed of linear electrical conductors 42, 43.This form of light panel, in association with a 4×10 array ofphotodiodes, linked by an optical connection as shown in FIG. 1, willprovide a one-bit digital cross-correlator with the output from thephotodiodes indicating the correlation or anti-correlation of thecompared signals at each cross-over of the matrix of conductors 42, 43.

A light panel for an m×n cross-correlator is illustrated in FIG. 5. Fromthe description above of the embodiment of FIG. 3, it will be clear thata cross-correlator incorporating the panel of FIG. 5 will

(a) be able to provide an indication of either correlation oranti-correlation by selection of the settings of the m linked switches53, to enable offset and drift in the photodiode array to becompensated; and

(b) provide true XOR function outputs if linked switches 54 and 56 areoperated at twice the sample frequency of the signals being analyzed, orat the sample frequency but with a quarter cycle phase difference.

References 51, 52 and 55 are to logic inverting gates. Note that drivers57 and 58 are required for each row and column in the matrix ofconductors, which have a LED/resistor combination 50 connected acrosseach cross-over point.

In each of these embodiments, the read-out from the photodiode array canbe displayed or processed by any suitable means, or stored for futureprocessing. It is expected that the output of the photodiode arrays ofcorrelators constructed in accordance with the present invention, if thearray contains a substantial number of elements, will be interfaced to acomputer for analysis purposes.

To test the present invention, an array of 128 light emitting diodes waswired as an autocorrelator and imaged on to a 128-element photodiodearray. Square waves of different frequencies were used as input signals.The arrangement performed as a 128 point autocorrelator.

In another test of the present invention, a 128 element, 16×8 matrixarray of LEDs was wired as a cross-correlator in accordance with thearrangement illustrated in FIG. 5. This light panel was used to processthe signals that would be obtained from a T-shaped array of receivingaerials forming a radio telescope, with 16 aerials in the long arm and 8aerials in the short arm. The outputs of the LEDs were imaged on to aphotographic film for the periods of measurement. The film, whendeveloped, showed clearly the integrated signal from each LED in thelight panel, and faithfully reproduced the spatial frequency fringepattern received by the array of aerials.

A 64×64 matrix array of LEDs has also been constructed in accordancewith the arrangement of FIG. 5. This has functioned effectively as a64×64 (=4096) channel correlator.

Earlier in this specification, reference was made to alternatives to theLED/resistor combination for the light panels. One of the alternativesmentioned was a neon discharge lamp. Panels incorporating neon lamps atthe cross-overs of the matrix of conductors are easy to fabricate andhave the advantage that the neon lamp flashes when the potentialdifference across its terminals is sufficient, irrespective of polarity.However, the required voltage difference is generally between 100 and200 volts. Series current limiting resistors or charge limitingcapacitors would be needed at each neon lamp location for fully paralleloperation.

Another alternative to the LED array is a liquid crystal sheet with alamp located behind it. With a sheet of liquid crystal material,electrodes can be inserted at appropriate locations and wired up asindicated for the LED light panels, and when there is a voltagedifference between the two adjacent electrodes, the material between theelectrodes changes from black to clear, so that light from the lamp istransmitted through the sheet at that location. The regions of liquidcrystal material between each pair of electrodes thus becomes a lightmodulating element. The light flash obtained when a region of liquidcrystal material becomes transparent can be focused on to a photodiodeas in the case of the LED array light panel to form a correlator. Thereis no requirement for a series resistor with this alternative, for theliquid crystal material is voltage sensitive.

A variation of the liquid crystal alternative is the use of singlecrystals of a material which exhibits the longitudinal Pockels effect.One example of this is single crystal DKDP (deuterated potassiumdi-hydrogen phosphate). Again the panel element will be opaque ortransparent at the cross-over locations, according to the voltage statesat the ends of the crystal. Again, no series resistor is required.

Some variations in the use of correlators incorporating the light panelwill now be described.

Quantization of an analogue signal for one-bit correlation necessarilyintroduces a distortion to the correlation function (which cansubsequently be corrected) and a sensitivity loss. The sensitivity losscan be recovered, at least in part, by correlating samples which havetwo-, three- or many-bit representation, by applying to the one-bitquantized signal a modulation which limits the time it is applied to afraction of the sample time which is proportional to the analogue signallevels. That is, the quantized one-bit signal is pulse width modulated.In fact, an analogue signal can be correlated without loss ofsensitivity if pulse width modulation is used to impose the analoguemodulation to the columns of a rectangular matrix array, and directcurrent or voltage modulation is applied to the rows. Those skilled inthis art will recognise that further complexities of current modulationof rows and pulse width modulation of columns can be used to achieveanalogue/analogue cross-correlation.

INDUSTRIAL APPLICABILITY

One large application of correlation in science is to correlate flashesof light represented by the logic state "1. " This application differsfrom the above description in that the logical AND function is required.To illustrate an AND function in the light panels illustrated in FIGS.1, 3, 4 and 5, an LED in the panel must flash whenever the logical state1 appears on both of its inputs. This is easily obtained. Indeed, itresults in a simplification of the panels illustrated in FIGS. 3 and 5,for the necessary modification is the removal of the polarity reversinggates and switches.

It is also possible to perform spatial autocorrelation of ablack-and-white picture using the present invention with the lightpanels modified to display an AND logic event, as described in the lastpreceding paragraph. Spatial autocorrelation is often useful in imageprocessing, image analysis, and speckle interferometry in astronomy. Theautocorrelation of a picture measures the number of times bright picturepoints have the same separation and orientation. The result is adistribution of separations and orientations with the value at eachpoint the number of times that one occurred in the given picture. Theimage to be processed is read out by a TV type of camera as a series oflines across the image. If the picture is black, the output is "0"; ifthe picture is white, the output is "1". A gap equal to one line timemust be present between reading successive lines and a gap equal to oneframe time must be present between reading successive frames. The lightpanel is then used to take the temporal autocorrelation of the one-bitsignal. Bright picture points of a given separation and orientationalways have the same gap in time on the signal and all separations andorientations have a unique gap or delay. The autocorrelation thereforecontains all the information required. If the array of LEDs (or otherelements) in the light panel of the correlator is made with each rowcontaining as many elements as two line times (i.e. one line plus itsgap), then the true two-dimensional distribution is displayed directly.The light panel, unlike other approaches to autocorrelation of apicture, can perform the calculation at video rates of many megahertz orfull picture frames at 100 frames per second or more.

Another application of the correlators and the method of the presentinvention is to the direct spectral analysis of a temporal signal. Thecosine spectrum of a signal V(t) which varies with time is given by therelationship:

    1/T .sub.O.sup.T V(t) cos (wt)dt

That is, it is the average of the product of the signal and a cosine offrequency w for different values of w. If a light which varies inintensity as cos wt is modulated (multiplied) by V(t), as a function oftime, then the mean light output is one component of the spectrum. Thelight panel of the present invention offers a unique way of obtainingthe complete spectrum simultaneously. The required information isobtained using a correlator having a light panel in which all the lightemitting elements vary in output of photons as cos wt, but each lightemitting element has its own unique value for w. Then V(t) modulates thelight from the entire panel (by pulse width, or voltage, or currentmodulation approaches, or by overall shutter) and a complementaryphotodiode array measures the mean light level (plus a fixed lightoffset) of each element of the light panel. Precisely this arrangementis obtained if a light panel of a correlator constructed in accordancewith the present invention is wired to perform autocorrelation of a timesignal having a frequency which is sweeping linearly with time. Then ateach delay, two frequencies are compared. They beat together at thelight point to produce a cos (Δw)t variation. Note that Δw isproportional to delay for autocorrelation.

The main complication with such an arrangement is that for one-bitsignals, a triangular rather than cosine variation of frequency w isobtained. One-bit times analogue or other approaches (see above) can beused to provide more exact spectra, or deconvolution can be used toconvert the triangular derived spectrum.

Yet another example of the use of correlators constructed in accordancewith the present invention is in the direct spatial frequency analysisof a two-dimensional picture. This further application is possible whenautocorrelation is taken of the sweeping signal on a light panelorganized in two dimensions, as for the spatial correlation application.The autocorrelation of a cosine wave is also a cosine wave and when theautocorrelation is folded into the raster pattern of a two dimensionalpanel, it appears as a spatial wave distribution of light of someorientation and wavelength. A different frequency gives anautocorrelation which appears as a different spatial frequency. Indeed,if the incoming signal to a light panel wired as an autocorrelator isswept in frequency, the light panel appears to sweep through all thepossible spatial frequencies, and thus to scan through all spatialfrequencies sequentially. If a transparency is placed on the panel, thenthe scene of the transparency is modulated (multiplied) by the spatialfrequency. The total light transmitted through the transparency is thenproportional to that spatial frequency content of the transparency'sscene. Only a single light detector (photodiode) is required in thisapplication. As the input signal sweeps, the single photodiode output isa scan through all spatial frequency components. If the output isappropriately scanned over a screen, then the true two dimensionalspatial frequency spectrum of the scan can be directly displayed. Itshould be noted that any offset levels can be simply cancelled bysubtraction, and that the sine wave spectrum is available by simpleadditions to the autocorrelator.

These examples of the use of the present invention are not exhaustive.

I claim:
 1. A correlator for performing autocorrelation, comprising:(a)a linear array of illumination control elements each having a firstoptical state in which a first light output is provided and a secondoptical state in which a second light output is provided, each saidelement changing from its first optical state to its second opticalstate when a voltage is applied across it, each element being adapted toreceive two voltage signals, the optical state of each element at anyinstant being dependent on the relative values of the voltage signals;(b) means for applying a voltage signal from a single signal source toone input of each element in the linear array, said voltage signal fromsaid single signal source representing a signal to be autocorrelated;(c) means for applying said signal from said single signal source, aftera delay thereof by a respective integral number of time delay periods,to the other input of each element in the linear array; and (d) an arrayof photodiodes, each photodiode receiving light from a respective one ofsaid elements and providing an output signal responsive to said receivedlight, the output signals of said photodiodes representing anautocorrelation of said signal to be correlated.
 2. A correlatorcomprising:(a) a rectangular m×n array of illumination control elementseach having a first optical state in which a first light output isprovided and a second optical state in which a second light output isprovided, each said element changing from its first optical state to itssecond optical state when a voltage is applied across it, each elementbeing adapted to receive two voltage signals representing signals to becorrelated, the optical state of each element at any instant beingdependent on the relative values of the voltage signals; each elementbeing located at a cross-over point of an m×n matrix formed by aplurality of linear electrical conductors, each conductor beinginsulated from each other conductor in the matrix; each element havingtwo input terminals, each of which is connected to a respective one ofsaid conductors at the cross-over point where the element is located; mand n being positive integers greater than unity; and (b) acomplementary array of photodiodes, each photodiode receiving light froma respective one of said elements, the outputs of said photodiodesrepresenting a correlation of said signals to be correlated.
 3. Acorrelator comprising:(a) a non-rectangular array of illuminationcontrol elements each having a first optical state in which a firstlight output is provided and a second optical state in which a secondlight output is provided, each said element changing from its firstoptical state to its second optical state when a voltage is appliedacross it, each element being adapted to receive two voltage signalsrepresenting signals to be correlated, the optical state of each elementat any instant being dependent on the relative values of the voltagesignals; each element being located at a cross-over point of a matrix ofelectrical conductors, each conductor being insulated from each otherconductor in the matrix, and each element being connected so that itstwo input signals are signals on respective ones of the conductors atthe cross-over points where the element is located; and (b) acomplementary array of photodiodes, each photodiode receiving light froma respective one of said elements, the outputs of said photodiodesrepresenting a correlation of said signals to be correlated. 4.Apparatus as defined in claim 1 claim 2 or claim 3, in which the lightoutput of each illumination control element is focused by optical meanson to its associated photodiode.
 5. A correlator as defined in claim 1,in which each of said signals is a quantized one-bit voltage signal. 6.A correlator as defined in claim 2 or claim 3, in which said two voltagesignals are applied to respective ones of said linear electricalconductors, and wherein the signal applied to each conductor is arespective quantized one-bit voltage signal.
 7. An auto-correlator asdefined in claim 1 or claim 5, in which said means for applying a commonvoltage signal to one input of each element comprises a logic invertinggate and means for permitting either the signal or the inverse of saidsignal to be applied to the elements of the linear array.
 8. Anauto-correlator as defined in claim 7, further characterised by a switchbeing positioned between said logic inverting gate and said elements ofthe linear array, to enable either correlation or anti-correlation to beindicated by the photodiode array.
 9. An auto-correlator as defined inclaim 7 or claim 8, including, in association with each input to eachelement, a circuit comprising two conducting lines in parallel, afurther logic inverting gate being included in one of said lines inparallel, and a switch to connect one of said lines in parallel to therespective input of an element.
 10. A cross-correlator as defined inclaim 2, wherein said m×n array includes m rows of conductors eachhaving an input end, and in which the input end of each of said m rowsof conductors is connected to the output of a respective first circuitconsisting of(a) a first pair of conducting lines, connected inparallel, one end of each line of said first pair being connected to aninput of said first circuit, one of said first pair of conducting linesincluding a logic inverting gate; (b) a second pair of conducting lines,connected in parallel, one end of each line of said second pair ofconducting lines being connected to an output of said first circuit, oneof said second pair of conducting lines including a logic invertinggate; and (c) first and second switches being disposed respectivelybetween said first pair of conducting lines and said second pair ofconducting lines; said first switch being adapted to select which one ofsaid first pair of conducting lines will be included in the path of anincoming signal; said second switch being adapted to select which one ofsaid second pair of conducting lines will be included in the path of anincoming signal; and the input end of each of said n columns ofconductors being connected to the output of a respective second circuitcomprising(i) a third pair of conducting lines, connected in parallel,one of said third pair of conducting lines including a logic invertinggate, and (ii) a third switch connecting one of said third pair ofconducting lines into the path of an incoming signal; furthercharacterized in that all said first switches are linked to be operatedsimultaneously, all said second switches are linked to be operatedsimultaneously, all said third switches are linked to be operatedsimultaneously, and second switches are linked for simultaneousoperation with said third switches.
 11. A cross-correlator as defined inclaim 10, in which each of said elements consists of a light emittingdiode in series with a resistor, said cross-correlator including m+nsignal drivers, each said driver being connected between one of saidfirst or second circuits and its respective electrical conductor.
 12. Acorrelator as defined in claim 1, claim 2 or claim 3, including means toread out the array of photodiodes.
 13. A correlator as defined in claim1, claim 2 or claim 3, including an interface between the photodiodesand a computer.
 14. A method of correlating a plurality of pairs ofquantized electrical signals, comprising the steps of:(a) applying eachpair of quantized signals to the two terminals of a respective elementin an array of illumination control elements each having a first opticalstate in which a first light output is provided and a second opticalstate in which a second light output is provided, each element havingtwo terminals and changing from its first optical state to its secondoptical state when a voltage difference is applied across its twoterminals; (b) observing the optical state of each element in the arrayby focusing the light from each element onto a respective photodiode inan array of photodiodes which is complementary to said array ofelements; and (c) reading out the information contained in saidcomplementary array of photodiodes, said information contained in saidcomplementary array of photodiodes representing a correlation of saidplurality of pairs of quantized electrical signals.
 15. A method asdefined in claim 14, in which one of the signals of said pair of signalsis a signal from a single source, and the other signal of said pair ofsignals is the signal from the single source which has been delayed byan integral number of units of time delay (τ), whereby the method is amethod of autocorrelation.
 16. A method as defined in claim 14, in whichthe array of elements is an m×n array, and each element is located atthe cross-over point of an m×n matrix of conductors with its terminalsconnected to respective ones of the two conductors at the cross-overpoint, whereby said monitoring of the optical state of each element (10)is effected to perform cross-correlation of the signals applied to theconductors; where m and n are positive integers.
 17. A method as definedin claim 14, claim 15 or claim 16, in which at least one of the signalsto be correlated is a many-bit quantized signal, said method beingfurther characterised in that it includes a preliminary step ofconverting the or each many-bit quantized signal with pulse widthmodulation of the one-bit quantized signal the width of said pulse widthmodulation being directly proportional to the instantaneous level, atthe time of sampling, of the or the respective many-bit quantizedsignal.
 18. A method as defined in claim 14, in which at least onesignal to be correlated is derived from an analogue signal, said methodbeing further characterised in that it includes a preliminary step ofconverting said or each analogue signal into a one-bit quantized signalwith pulse width modulation of the one-bit quantized signal, the widthof said pulse width modulation being directly proportional to theinstantaneous value, at the time of sampling, of the or the respectiveanalogue signal.